This invention relates to an imprroved drive circuit for an electroluminescent (EL) cell, panel, or lamp.
Generally, an electroluminescent cell or lamp or panel which is known in the art includes a transparent, electrically conducting front electrode, a thin binder layer of transparent thermoplastic material loaded with an electroluminescent phosphor powder forming a matrix, and a reflecting, electrically conducting back electrode. Light is emitted through the front electrode when an alternating current electric field is applied across the phosphor powder. The electric field which is produced by a driving circuit excites the phosphor to luminesce.
Light output from phosphors in the matrix is composed of individual pulses of light generated at each voltage pulse such that light emission intensity is proportional to the voltage--pulse frequency. U.S. Pat. No. 4,238,793, issued to Hochstrate and assigned to Timex Corporation, discloses a driving circuit for exciting phosphors in EL lamps to luminesce by providing a low duty cycle pulsed high frquency potential. U.S. Pat. No. 4,253,097, issued to Hochstrate and assigned to Timex Corporation, also discloses a drive circuit for activating electroluminescent panels. Both patents are incorporated herein by reference.
A low-voltage, battery-supplied power source for pulse driving a novel electroluminescent lamp is discussed in copending U.S. patent application, Ser. No. 362,708, filed on Mar. 29, 1982. The drive circuit includes an oscillator for periodically providing an alternating electric field to excite the phosphor in the EL lamp to luminesce. The oscillator in the pending patent application provides low duty cycle pulses of less than 25 percent at a predetermined high frequency to a switch which includes two transistors. The switch produces an output pulse while it is "on" for the duration of each pulse waveform produced by the oscillator. The switch turns "on" an inductor for the duration of each switch output pulse. The energy that builds up in the field of the inductor when it is "on" is applied across the EL lamp in the manner described in the pending application in order to excite the phosphor to luminesce.
The 555 integrated circuit timer developed by Signetics, Inc. is a new generation of timer chips having, as an important characteristic, frequency and period stability. In other words, it does not change parameters merely because the supply voltage drops. The 555 uses the ratio of its supply voltage and certain of its IC terminal voltages so that, for example, the threshold potential is a fraction of the supply voltage.
The 555 may be used as a variable duty cycle astable multivibrator. Generally, the duty cycle, or duty factor, of an astable multivibrator relates the total HIGH (maximum such as +5 volts) output value to the total LOW (zero) output value. In the 555 astable circuit, the duty cycle is set by the two resistors in the frequency setting network portion of the circuit. By varying the ratio of the two resistors, the duty factor is easily changed.
The 555 can be made to interface with TTL, bipolar or CMOS families of IC logic circuits. The timing network (resistors and capacitor) in the 555 sets the operating frequency, the frequency setting network (resistors) sets the duty cycle and the supply voltage potential sets the amplitude of the 555 output signal. Additional aspects of the 555 timer are discussed in such texts as the "Guide To CMOS Circuits" by Howard M. Berlin, published by Howard W. Sams & Co., Inc., Indianapolis, Ind. (1979).